But the average time it takes for the symptoms of variant CJD to occur after initial infection (the incubation period) is still unclear.

The incubation period could be very long (more than 10 years) in some people, so those exposed to infected meat before the food controls were introduced can still develop variant CJD.

The prion that causes variant CJD can also be transmitted by blood transfusion, although this has only happened 4 times in the UK.

In 2014, there were no recorded deaths from variant CJD in the UK.

Familial or inherited CJD

Familial CJD is a very rare genetic condition where one of the genes a person inherits from their parent (the prion protein gene) carries a mutation that causes prions to form in their brain during adulthood, triggering the symptoms of CJD.

It affects about 1 in every 9 million people in the UK.

The symptoms of familial CJD usually first develop in people when they're in their early 50s.

In 2014, there were 10 deaths from familial CJD and similar inherited prion diseases in the UK.

Iatrogenic CJD

Iatrogenic CJD is where the infection is accidentally spread from someone with CJD through medical or surgical treatment.

For example, a common cause of iatrogenic CJD in the past was growth hormone treatment using human pituitary growth hormones extracted from deceased individuals, some of whom were infected with CJD.

Synthetic versions of human growth hormone have been used since 1985, so this is no longer a risk.

Iatrogenic CJD can also occur if instruments used during brain surgery on a person with CJD aren't properly cleaned between each surgical procedure and are reused on another person.

But increased awareness of these risks means iatrogenic CJD is now very rare.

In 2014, there were 3 deaths from iatrogenic CJD in the UK caused by receiving human growth hormone before 1985.

How CJD is treated

There's currently no cure for CJD, so treatment aims to relieve symptoms and make the affected person feel as comfortable as possible.